Redox Behavior of CuZSM-5 Catalysts:  FTIR Investigations of Reactions of Adsorbed NO and CO

Jang, Heejung; Hall, and W. Keith; d’Itri, Julie L.

doi:10.1021/jp952525x

Cited by 102 publications

(102 citation statements)

References 42 publications

Supporting

Mentioning

100

Contrasting

Order By: Relevance

“…[4][5][6][7] The expected presence of such species, together with electron spin resonance and thermogravimetric studies, 7 suggest that redox cycles are responsible for NO decomposition turnovers. Infrared spectra of adsorbed CO and NO provided indirect evidence for the reversible cycling between Cu 2+ and Cu + during switching between O 2 and He streams at 773 K. 8 These cycles were confirmed by X-ray photoelectron (XPS) and X-ray absorption (XAS) spectroscopic studies. 9 The parallel increase in the concentration of Cu + species detected by XAS and in the rate of NO decomposition was used as evidence for Cu + as a reaction intermediate.…”

Section: Introductionmentioning

confidence: 85%

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Costa

Modén

Meitzner

et al. 2002

Phys. Chem. Chem. Phys.

152

145

View full text Add to dashboard Cite

The number and type of Cu 2+ species present on O 2 -treated Cu-ZSM5 catalysts (Si/Al ¼ 13.1-14.6) with varying Cu/Al ratios (0.12-0.60) were measured using temperature-programmed reduction in H 2 or CO and desorption of O 2 with He as the carrier. The effluent stream was monitored using mass spectrometry and the structure and oxidation state was determined in parallel by X-ray absorption spectroscopy. Isolated Cu 2+ monomers and oxygen-bridged Cu 2+ dimers interacting with Al-Al next nearest neighbor pairs were the predominant Cu species on these catalysts. The fraction of Cu present as dimers increased from 0.46 to 0.78 as Cu/Al ratios increased from 0.12 to 0.60, as expected from the decreasing average Cu-Cu distance with increasing Cu content. In contrast, monomers reached a plateau of $0.15 Cu 2+ /Al, suggesting that only some Al-Al pairs can interact with small Cu 2+ monomer structures, while a much larger fraction can bind with larger oxygen-bridged Cu 2+ dimers. The measured distribution of Cu dimers and monomers is consistent with the number and bond distances of Al-Al pairs for the Si/Al ratio in these ZSM5 samples. The distributions of Cu species obtained from the amount of CO 2 formed (from CO), the amount of H 2 O formed (from H 2 ), and the amount of CO adsorbed after reduction in CO are in excellent agreement. The number of oxygen atoms removed as O 2 was significantly smaller than that removed with H 2 or CO, suggesting that only proximate Cu dimers autoreduce via recombinative desorption steps. NO decomposition turnover rates (normalized per Cu dimer) were nearly independent of Cu content, except at the lowest Cu/Al ratio, consistent with the involvement of Cu dimers as the active Cu species in NO decomposition redox cycles on Cu-ZSM5. Multiple O 2 and CO 2 peaks during desorption and reduction in CO suggest the presence of Cu dimers with varying oxygen binding energy and reactivity. The Cu dimers initially formed at low Cu/Al contents during exchange are less reducible, consistent with their lower NO decomposition turnover rates.

show abstract

Section: Introductionmentioning

confidence: 85%

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Costa

Modén

Meitzner

et al. 2002

Phys. Chem. Chem. Phys.

152

145

View full text Add to dashboard Cite

show abstract

“…Most of the cases concern Cu ϩ cations in zeolites, where a band assigned to adsorbed dinitrogen has been detected in the 2299-2290-cm Ϫ1 region [32,[129][130][131]. Most of the cases concern Cu ϩ cations in zeolites, where a band assigned to adsorbed dinitrogen has been detected in the 2299-2290-cm Ϫ1 region [32,[129][130][131].…”

Section: Identification Of N X O Y Species 75mentioning

confidence: 99%

Identification of Neutral and Charged N_xO_ySurface Species by IR Spectroscopy

2000

View full text Add to dashboard Cite

“…12 These were the only infrared bands detected at 673 K. Infrared studies have also confirmed that NO adsorbs on Cu + 5,13-18 and suggested that Cu + /Cu 2+ redox reactions occur as conditions change from reducing to oxidizing. 19 NO decomposition appears to occur via the initial formation of N 2 O followed by its decomposition. 26 N 2 O and small amounts of N 2 are typically detected during temperature-programmed desorption of preadsorbed NO on Cu-ZSM5.…”

Section: Introductionmentioning

confidence: 99%

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

et al. 2002

View full text Add to dashboard Cite

The identity of reactive intermediates and of active sites and the details of redox cycles and oxygen removal pathways during NO decomposition on well-characterized Cu-ZSM5 were examined by combining previous spectroscopic and steady-state kinetic studies with measurements of the rate of evolution of NO, N 2 O, NO 2 , N 2 , and O 2 during isothermal and nonisothermal kinetic transients. The oxygen coverages measured during reversible isothermal switches from He to NO/He mixtures showed that NO decomposition involves bimolecular reactions of two NO molecules adsorbed on vicinal Cu + species with the formation of N 2 O as the initial product near ambient temperature. These vicinal Cu + species form via oxygen removal from {Cu2+ using NO 2 as an oxygen carrier among distant oxidized dimers. Adsorbed nitrate (NO 3 *) is the kinetically relevant intermediate in the formation of O 2 during NO decomposition. This NO 3 * decomposition reaction is one of the steps involved in the equilibrated formation of NO 2 observed during NO decomposition. These NO-mediated oxygen removal pathways, in which NO acts both as a reductant and as an oxidant, are significantly more rapid than recombinative desorption steps. The desorption of products and of unreacted NO during reactions of preadsorbed NO with increasing catalyst temperature confirmed the bimolecular nature and the low activation energy for N 2 O formation from NO. The facile nature of this reaction and the unfavorable NO adsorption thermodynamics as temperature increases combine to give the observed decrease in NO decomposition rates at high temperatures. These findings are consistent with some reported mechanism-based steady-state rate expressions and with previous infrared detection of the reaction intermediates proposed here based on isothermal and nonisothermal transients. These pathways appear to be relevant also to N 2 O decomposition reactions, which occur after the initial formation of N 2 O from NO and involve reactions of N 2 O with {Cu2+ and removal of oxygen via NO-mediated desorption pathways. This study brings consensus and some clarification into the mechanistic details for NO and N 2 O decomposition reactions and resolves some remaining discrepancies and some contradictory conclusions in previous reports.

show abstract

Redox Behavior of CuZSM-5 Catalysts: FTIR Investigations of Reactions of Adsorbed NO and CO

Cited by 102 publications

References 42 publications

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Identification of Neutral and Charged N_xO_ySurface Species by IR Spectroscopy

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

Contact Info

Product

Resources

About

Redox Behavior of CuZSM-5 Catalysts: FTIR Investigations of Reactions of Adsorbed NO and CO

Cited by 102 publications

References 42 publications

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Spectroscopic and chemical characterization of active and inactive Cu species in NO decomposition catalysts based on Cu-ZSM5

Identification of Neutral and Charged NxOySurface Species by IR Spectroscopy

Transient Studies of Oxygen Removal Pathways and Catalytic Redox Cycles during NO Decomposition on Cu−ZSM5

Contact Info

Product

Resources

About

Identification of Neutral and Charged N_xO_ySurface Species by IR Spectroscopy